1. Field of the Invention
The present invention relates to semiconductor devices and more particularly to a phase-change memory device.
2. Description of the Related Art
Ongoing developments in electronic devices such as mobile telephones, personal media players, and computers, result in an increasing demand for semiconductor memory devices characterized by a high operating frequency, greater integration density, nonvolatile data storage, and low power consumption. By many measures, this constellation of demands remains unmet by contemporary memory devices, (e.g., dynamic random access memories (DRAMs), static random access memories (SRAMs), flash memories, etc.).
For instance, DRAMs and SRAMs provide only volatile data storage that requires maintenance of constant power. It has also proved difficult to increase the integration density of SRAMs because each constituent memory cell is formed from six (6) transistors. Contemporary flash memory devices, while nonvolatile in their data storage, are relatively slow in their operating frequency.
As a result of these ongoing deficiencies, phase-change random access memories (PRAMs) have become a subject of intense research and development, because in theory they offer the combination of low power consumption, high operating frequencies, and nonvolatile memory storage.
PRAMs are memory devices using one or more phase-change materials. This material acts as a variable resistor within the data storage element of a PRAM. In one view, PRAMs may be formed very much like conventional DRAMs save the dielectric material used to form the DRAM capacitor is replaced with a phase-change material. The resistance of common phase-change materials varies with a “phase” or state of the material. For example, a phase-change material may have a higher resistivity in a crystalline phase as compared with an amorphous phase. By sensing variations in a voltage and/or a current dictated by this variable resistance, it is possible to differentiate data states (e.g., logical values of ‘1’ or ‘0’). One promising phase-change material is formed with a chalcogenide containing germanium (Ge), antimony (Sb), and tellurium (Te).
FIG. 1 is a sectional schematic illustrating a general structure of a PRAM. Referring to FIG. 1, a phase-change material pattern 40 and a top electrode 45 are sequentially stacked on a semiconductor substrate 10 including a bottom electrode. The top electrode 45 is electrically connected to an interconnection 80 through a bottom electrode contact 75 penetrating an interlevel (or interlayer) insulation layer 70.
The phase-change material pattern 40 has a phase-change region “P” that transforms into an amorphous phase or a crystalline phase under the influence of applied thermal energy. The phase-change region P is formed within the phase-change material pattern 40 in contact with a bottom electrode 35.
While write and read operations proceed in the illustrated PRAM, titanium atoms (e.g., from top electrode 45) are diffused into the phase-change material pattern 40. Such diffusion of titanium atoms varies a defined composition ratio for the components forming the phase-change material pattern 40. In particular, if the titanium atoms diffuse into the phase-change region P, the PRAM will be unable to continue proper operation. This possible result is further explained in the graphs of FIGS. 2A, 2B and 2C.
FIGS. 2A, 2B, and 2C are graphs showing relative quantities of titanium as a function of position and illustrate the effect of titanium diffusion within a phase-change memory device. FIG. 2A illustrates conditions that exist within the exemplary PRAM upon initial operation. FIG. 2B illustrates conditions that exist following repeated execution of write and read operations in the PRAM. FIG. 2C illustrates conditions that exist when the exemplary PRAM has reached the end of operational lifespan. The horizontal axes in these graphs indicate relative diffusion positions between points “A” and “B” in the phase-change material of FIG. 1. In one example corresponding to the graphs of FIGS. 2A, 2B and 2C, 50 nm position is located in an upper part of the phase-change material pattern 40 near the top electrode 45 and the 100 nm position is located in a lower part of the phase-change material pattern 40 near the bottom electrode 35. The vertical axes of these graphs indicate relative quantities of materials existing at their corresponding positions in the phase-change material pattern.
Referring to FIG. 2A, before initiating read/write operations in the PRAM, the relative concentration of titanium atoms within the phase-change material 40 is small. In contrast and as illustrated in FIG. 2B, this concentration increases with repeated execution read/write operations in the PRAM as titanium atoms migrate from the top electrode 45 into the phase-change material pattern 40. Of note, the concentration of titanium atoms increases in the upper part of the phase-change material 40, but the lower part of the phase-change material 40 remains relatively free of diffused titanium atoms. Referring now to FIG. 2C, at the end of the operational lifespan of the PRAM, the concentration of diffused titanium atoms actually falls in the upper part of the phase-change material 40, but markedly increases in the phase-change region P. Thus, as illustrated by the example shown in FIGS. 2A, 2B and 2C, during operation of conventional PRAMs, titanium atoms tend to diffuse from an electrode into a phase-change region. At higher concentrations of titanium atoms within the phase-change region, the performance capabilities and overall reliability of the PRAM become impaired.